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66                            HEART OF MONOTREMATA                        CHAP.
stance in the heart of the Rabbit. The heart of the Monotre-
inata presents differences of some importance from those of other
Mammalia; the modern knowledge of the Monotrematous heart
is mainly due to Gegenbaur1 and Lankester,2 in whose memoirs
references to the older literature will be found. The principal
features of interest in which the heart of the Monotremata differs
from that of the higher Mammalia are these. When the two
ventricles are cut across transversely, the cavity of the right is
seen to be wrapped round that of the left in a fashion precisely
like that of the bird's heart; on the other hand in the higher
mammal the two cavities lie side by side. The main difference
"between Monotremes and other Mammals concerns the right
auriculo - ventricular valve. The differences which it presents
from the corresponding structure of the rest of the Mammalia
are two : in the first place, the valve itself does not com-
pletely surround the ostium; it is only developed 011 one side;
the septal half (i.e. that turned towards the interventricular
septum) is either entirely absent or more generally represented
by a small bit of membrane; nevertheless I found3 recently in
an OrnitJiOThynchus heart a complete septal half to the right
auriculo-ventricular valve. The second point of interest in
connexion with this valve is, that the musculi papillares instead
of ending in chordae tendineae attached to the free edge of the
val,r 4?.re directly attached to the valve, and in some cases pass
through its membranous flap, to be attached to its origin at the
boundary of the auricle and of the ventricle. The invading of
the valve-flap by muscle in this way is highly interesting, as it
recalls the heart of the bird and of the crocodile. Tho im-
perfect condition of the valve (from which, as has already been
etated, the septal half is as a rule nearly absent) is a point of
resemblance to the heart of the bird; the corresponding valve
of the crocodile's heart being complete.
There are also features in the system of arteries and veins
which are eminently distinctive of mammals. In the first place,
the aorta leaving the heart and conveying blood to the body
is only a half arch, and bends to the left side as seen in
Fig. 43. The right arid left halves are present in reptiles,
and meet "behind the heart. In the "bird the right liulf alone
1 Jen. Z&faxtor. ii. 1866, p, 365.
8 JProc. Moot* Sec. 1888, p. 8.                        a Proc. Zo&l. &<K. 1891, p. 715.